The invention relates to a photon-counting imaging device for single x-ray counting. Further, the invention relates to a photon-counting imaging device with a fast region of interest data evaluation.
X-ray diffraction patterns are useful in the analysis of molecular structures, such as protein and virus molecules, and require photon counting imaging devices. Especially, protein and virus crystallography imposes stringent requirements on x-ray detectors, particularly where the x-ray source is high flux synchrotron radiation that enables an experiment to be done rapidly. Furthermore, an important and developing field is time-resolved diffraction experiments using synchrotron radiation, such as crystallography and/or powder diffraction analysis. Monitoring a time-dependent reaction in a sample, i.e. a crystal or a powder, can elucidate the time-dependent crystal/molecular changes that occur in a chemical reaction as well. High time resolution speed is often critical in such monitoring.
In the literature, a high speed crystallography detector is disclosed by the U.S. Pat. No. 5,629,524 and a solid-state image sensor with focal-plane digital photon-counting pixel array is disclosed by the U.S. Pat. No. 5,665,959. The latter patent describes and illustrates a focal-plane array comprising an array of N×N photodetector diodes connected to a digital photon-counting means for ultrlow level image light detection and digital image pixel readout means for each pixel comprising separate CMOS buffer amplifiers that exhibit the following characteristics: low power (<1 μW per pixel average), high photoelectron charge to voltage conversion gain (≈1 mV/e−), low noise (<1 e−), small cell pitch (<30 μm), easy scalability (to 10 μm), self biasing capability, sufficient gain uniformity (˜10%) for multiple event discrimination, and bias current programmability. Any incident photon during the sampling period generates a photoelectron at the output of the detector diode connected to the input of the amplifier. That photoelectron changes the potential of the buffer amplifier's input capacitance. This change in potential causes the high-gain buffer amplifier to present a sufficiently large voltage change at the output of the amplifier to be above the system noise level.
The drawback of this disclosure remains substantially in the design of this photon counting device that is dedicated to detect with brilliant sensitivity a single photon having its energy in the range of the visible light (several eV). This device can therefore be used for infra-red binoculars or for space-based telescopes and spectrometers. The electronic circuitry is therefore that sensitive that an incident photon is amplified in order to saturate the buffer of the amplifier. An additional incident photon occurring in the same photodetector diode within the same sampling period as the first incident photon therefore can not be detected unless the buffer is reset. This photon detecting device is therefore completely useless for the above-mentioned purposes of x-ray photon detection.
Nevertheless, the general design of the semiconductor chip is preferably a hybrid using a separate semiconductor material for two chips selected to be optimum for the photovoltaic type of detector diodes in one and the buffer amplifier and multiplexing circuit in the other chip bump bonded to the first to make connections between the output interface of the detector diodes on one chip and the input interface of the buffer amplifier on the other chip with the photodetector diodes buffer amplifier in one semiconductor chip and the multiplexing means and digital counters on the second semiconductor chip bump bonded to the first. As disclosed in the U.S. Pat. No. 5,629,524 a suitable material for the electrical bump connection is Indium. But even this device for x-ray photon detection can not be used in high dynamic investigation since the electronic circuitry is limited due to the switching dead time that is required to integrate the charge of the photo electrons subsequently to a chain of capacitors (referred to as a M-bit shift register) which have to be read out afterwards serially due to its chain-like arrangement. It could be easily understood that the performance of this circuitry is limited to its switching intervals for charging and scanning the capacitors.
Another prior art document worth to be mentioned is the U.S. Pat. No. 5,812,191 disclosing a semiconductor high-energy radiation imaging device having an array of pixel cells including a semiconductor detector substrate and a semiconductor readout substrate. The semiconductor detector substrate includes an array of pixel detector cells, each of which directly generates charge in response to incident high-energy radiation. The semiconductor readout substrate includes an array of individually addressable pixel circuits, each of which is connected to a corresponding pixel detector cell to form a pixel cell. Each pixel circuit includes charge accumulation circuitry for accumulating charge directly resulting from high-energy radiation incident on a corresponding pixel detector cell, readout circuitry for reading the accumulated charge, and reset circuitry for resetting the charge accumulation circuitry. Unfortunately, the accumulated charge is stored as analog data using a circuitry having two transistors, one transistor acting as the charge store while the other acts as a readout switch responsive to an enable signal. This design restricts the circuitry to allow individual addressing each pixel but only discharge the accumulated analog charge to an output line when activated by its respective enable signal. This circuitry does not enable any further manipulation of the pixel detector cells.
Another imaging device for imaging radiation according to the international patent application WO 98/16853 includes an image cell array. The image cell array includes an array of detectors cells which generates charge in response to instant radiation and an array of image cell circuits. Each image cell circuit is associated with a respective detector cell. The image cell circuit includes counting circuitry for counting plural radiation hits incident on an associated detector cell. Preferably, the image cell circuit includes threshold circuitry connected to receive signals generated in the associated detector cell and having values dependent on the incident radiation energy. The counting circuitry is then connected to the threshold circuitry for counting only radiation hits within a predetermined energy range or ranges. The electronic readout circuitry is designed to comprise a loadable shift register storing the data serially in a row that means the input data is the data from the previous pixel and the output delivers the actual data to the next pixel. The main drawback of this arrangement consists in the susceptibility to a failure of a complete row of the detector array if only one the readout circuitry in a row fails.
Furthermore, using x-ray diffraction for the analysis of the crystallographic structure of a sample a fast and reliable measurement procedure requires a comparably large detector array for covering a sufficient large spatial area. It is apparent from the required electronic equipment that an increasing number of detector arrays is followed by an increasing number of electronic equipment and/or a prolongated evaluation procedure.
Resuming the prior art documents it will be apparent that none of the documents disclose a photon counting imaging device that allow high readout performance und superior reliability of operation.
Therefore, it is the aim of the invention to increase the performance and the reliability of a complete photon-counting imaging device.